[C++20][Modules] Allow using stdarg.h with header units (#100739)

Summary:
Macro like `va_start`/`va_end` marked as builtin functions that makes
these identifiers special and it results in redefinition of the
identifiers as builtins and it hides macro definitions during preloading
C++ modules. In case of modules Clang ignores special identifiers but
`PP.getCurrentModule()` was not set. This diff fixes IsModule detection
logic for this particular case.

Test Plan: check-clang

---------

Co-authored-by: Chuanqi Xu <[email protected]>

Author: dmpolukhin

clang/lib/Serialization/ASTReader.cpp

@@ -1051,10 +1051,10 @@ IdentifierID ASTIdentifierLookupTrait::ReadIdentifierID(const unsigned char *d)
   return Reader.getGlobalIdentifierID(F, RawID >> 1);
 }
 
-static void markIdentifierFromAST(ASTReader &Reader, IdentifierInfo &II) {
+static void markIdentifierFromAST(ASTReader &Reader, IdentifierInfo &II,
+                                  bool IsModule) {
   if (!II.isFromAST()) {
     II.setIsFromAST();
-    bool IsModule = Reader.getPreprocessor().getCurrentModule() != nullptr;
     if (isInterestingIdentifier(Reader, II, IsModule))
       II.setChangedSinceDeserialization();
   }
@@ -1080,7 +1080,8 @@ IdentifierInfo *ASTIdentifierLookupTrait::ReadData(const internal_key_type& k,
     II = &Reader.getIdentifierTable().getOwn(k);
     KnownII = II;
   }
-  markIdentifierFromAST(Reader, *II);
+  bool IsModule = Reader.getPreprocessor().getCurrentModule() != nullptr;
+  markIdentifierFromAST(Reader, *II, IsModule);
   Reader.markIdentifierUpToDate(II);
 
   IdentifierID ID = Reader.getGlobalIdentifierID(F, RawID);
@@ -4547,7 +4548,7 @@ ASTReader::ASTReadResult ASTReader::ReadAST(StringRef FileName, ModuleKind Type,
 
       // Mark this identifier as being from an AST file so that we can track
       // whether we need to serialize it.
-      markIdentifierFromAST(*this, *II);
+      markIdentifierFromAST(*this, *II, /*IsModule=*/true);
 
       // Associate the ID with the identifier so that the writer can reuse it.
       auto ID = Trait.ReadIdentifierID(Data + KeyDataLen.first);
@@ -8966,7 +8967,8 @@ IdentifierInfo *ASTReader::DecodeIdentifierInfo(IdentifierID ID) {
     auto Key = Trait.ReadKey(Data, KeyDataLen.first);
     auto &II = PP.getIdentifierTable().get(Key);
     IdentifiersLoaded[Index] = ⅈ
-    markIdentifierFromAST(*this,  II);
+    bool IsModule = getPreprocessor().getCurrentModule() != nullptr;
+    markIdentifierFromAST(*this, II, IsModule);
     if (DeserializationListener)
       DeserializationListener->IdentifierRead(ID, &II);
   }

clang/test/Headers/stdarg-cxx-modules.cpp

@@ -0,0 +1,25 @@
+// RUN: rm -fR %t
+// RUN: split-file %s %t
+// RUN: cd %t
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header h1.h
+// RUN: %clang_cc1 -std=c++20 -emit-header-unit -xc++-user-header h2.h -fmodule-file=h1.pcm
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only main.cpp -fmodule-file=h1.pcm -fmodule-file=h2.pcm
+
+//--- h1.h
+#include <stdarg.h>
+// expected-no-diagnostics
+
+//--- h2.h
+import "h1.h";
+// expected-no-diagnostics
+
+//--- main.cpp
+import "h1.h";
+import "h2.h";
+
+void foo(int x, ...) {
+  va_list v;
+  va_start(v, x);
+  va_end(v);
+}
+// expected-no-diagnostics